feat(worker): 实现集群限流与自适应负载

保留平台模型 RPM、TPM 和并发策略语义，增加 PostgreSQL 集群级租约、饱和候选重选和多平台自动负载，避免突发任务固定等待首个平台。\n\n新增 Worker 实时负载采样、自适应 active/heavy 容量、心跳与管理端指标，并扩展本地 acceptance runner，覆盖三 Worker、同模型三平台 2/4/6 并发和 48 个带图视频突发任务。\n\n验证：go test ./...、go vet ./...、PostgreSQL 跨 Store 集成测试、gofmt、bash -n、ShellCheck 及本地集群 provider-burst 验收通过；48/48 成功，无越限、重复提交、重复计费、重复回调或终态资源泄漏。

apps/api/internal/store/admission_queue.go

@@ -747,14 +747,14 @@ SELECT COUNT(*)
 FROM gateway_concurrency_leases
 WHERE task_id = $1::uuid
   AND released_at IS NULL
-  AND expires_at > now()`, taskID).Scan(&count)
+  AND expires_at > statement_timestamp()`, taskID).Scan(&count)
 	return count, err
 }
 
 func resetTaskAdmissionToWaitingTx(ctx context.Context, tx pgx.Tx, input TaskAdmissionInput) (TaskAdmission, error) {
 	if _, err := tx.Exec(ctx, `
 UPDATE gateway_concurrency_leases
-SET released_at = now()
+SET released_at = statement_timestamp()
 WHERE task_id = $1::uuid
   AND released_at IS NULL`, input.TaskID); err != nil {
 		return TaskAdmission{}, err
@@ -862,7 +862,7 @@ func (s *Store) deleteTaskAdmissionOnce(ctx context.Context, taskID string) erro
 	}
 	if _, err := tx.Exec(ctx, `
 UPDATE gateway_concurrency_leases
-SET released_at = now()
+SET released_at = statement_timestamp()
 WHERE task_id = $1::uuid
   AND released_at IS NULL`, taskID); err != nil {
 		return err
@@ -884,7 +884,7 @@ SELECT id::text,
 FROM gateway_concurrency_leases
 WHERE task_id = $1::uuid
   AND released_at IS NULL
-  AND expires_at > now()
+  AND expires_at > statement_timestamp()
 ORDER BY scope_type, scope_key, id`, taskID)
 	if err != nil {
 		return nil, err
@@ -924,7 +924,7 @@ WHERE admission.mode = 'async'
         FROM gateway_concurrency_leases lease
         WHERE lease.task_id = admission.task_id
           AND lease.released_at IS NULL
-          AND lease.expires_at > now()
+          AND lease.expires_at > statement_timestamp()
       )
     )
   )
@@ -980,7 +980,7 @@ WHERE admission.mode = 'async'
         FROM gateway_concurrency_leases lease
         WHERE lease.task_id = admission.task_id
           AND lease.released_at IS NULL
-          AND lease.expires_at > now()
+          AND lease.expires_at > statement_timestamp()
       )
     )
   )
@@ -1006,6 +1006,29 @@ LIMIT $1`, limit)
 	return admissions, rows.Err()
 }
 
+// RequestWaitingTaskAdmissionReselect marks every queued task bound to a
+// saturated platform model so the dispatcher can route the next batch to a
+// different eligible candidate. The durable marker lets multiple dispatchers
+// observe the same decision without assigning tasks to a specific Worker.
+func (s *Store) RequestWaitingTaskAdmissionReselect(ctx context.Context, platformModelID string) (int64, error) {
+	result, err := s.pool.Exec(ctx, `
+UPDATE gateway_task_admissions admission
+SET reselect_requested_at = now(),
+    updated_at = now()
+FROM gateway_tasks task
+WHERE admission.task_id = task.id
+  AND admission.platform_model_id = $1::uuid
+  AND admission.mode = 'async'
+  AND admission.status = 'waiting'
+  AND task.status = 'queued'
+  AND task.next_run_at <= now()
+  AND admission.reselect_requested_at IS NULL`, platformModelID)
+	if err != nil {
+		return 0, err
+	}
+	return result.RowsAffected(), nil
+}
+
 // ListWaitingTaskAdmissionIDs returns one FIFO leader for every independent
 // platform-model queue, additionally requiring the task to lead its user-group
 // queue when one exists. It is used after capacity is released so API
@@ -1268,12 +1291,12 @@ func admissionScopeStatesTx(ctx context.Context, tx pgx.Tx, scopes []AdmissionSc
 		if scope.ConcurrentLimit > 0 {
 			if err := tx.QueryRow(ctx, `
 SELECT COALESCE(SUM(lease_value), 0)::float8,
-       COALESCE(MIN(expires_at), now() + interval '1 second')
+       COALESCE(MIN(expires_at), statement_timestamp() + interval '1 second')
 FROM gateway_concurrency_leases
 WHERE scope_type = $1
   AND scope_key = $2
   AND released_at IS NULL
-  AND expires_at > now()`, scope.ScopeType, scope.ScopeKey).Scan(&state.Active, &state.NextLeaseExpiration); err != nil {
+  AND expires_at > statement_timestamp()`, scope.ScopeType, scope.ScopeKey).Scan(&state.Active, &state.NextLeaseExpiration); err != nil {
 				return nil, err
 			}
 			state.Saturated = state.Active+scope.Amount > scope.ConcurrentLimit